1. Field of the Invention
This invention relates generally to a system and method for converting a steering angle command to a steering torque command and, more particularly, to a system and method for converting a steering angle command to a vehicle steering torque command that employs a steering model using data collected during development of the vehicle, where the model is updated in real time after deployment of the vehicle.
2. Discussion of the Related Art
The operation of modern vehicles is becoming more autonomous, i.e., vehicles are able to provide more driving control with less driver intervention. Cruise control systems have been on vehicles for a number of years where the vehicle operator can set a particular speed of the vehicle, and the vehicle will maintain that speed without the driver operating the throttle. Adaptive cruise control systems have been recently developed in the art where not only does the system maintain the set speed, but also will automatically slow the vehicle down in the event that a slower moving vehicle is detected in front of the subject vehicle using various sensors, such as radar, LiDAR and cameras. Modern vehicle control systems may also include autonomous parking where the vehicle will automatically provide the steering control for parking the vehicle, and where the control system will intervene if the driver makes harsh steering changes that may affect vehicle stability and lane centering capabilities, where the vehicle system attempts to maintain the vehicle near the center of the lane. Fully autonomous vehicles have been demonstrated that drive in simulated urban traffic up to 30 mph, while observing all of the rules of the road.
As vehicle systems improve, they will become more autonomous with the goal being a completely autonomously driven vehicle. Future vehicles will likely employ autonomous systems for lane changing, lane keeping, lane centering, passing, turns away from traffic, turns into traffic, etc. As these systems become more prevalent in vehicle technology, it will also be necessary to determine what the driver's role will be in combination with these systems for controlling vehicle speed, steering and overriding the autonomous system.
An examples of semi-autonomous vehicle control systems can be found in U.S. Pat. No. 8,190,330, issued May 29, 2012, titled “Model Based Predictive Control for Automated Lane centering/changing control systems,” assigned to the assignee of this application and herein incorporated by reference, which discloses a system and method for providing steering angle control for lane centering and lane changing purposes in an autonomous or semi-autonomous vehicle. U.S. Pat. No. 8,170,751, issued May 1, 2012, titled “Detection of Driver Intervention During a Torque Overlay Operation in an Electric Power Steering System,” assigned to the assignee of this application and herein incorporated by reference, discloses a system and method for controlling vehicle steering by detecting a driver intervention in a torque overly operation.
As mentioned, lane centering and/or lane keeping systems have been demonstrated in the art that cause the vehicle to autonomously follow the vehicle travel lane. Current lane centering and lane keeping controllers typically have a maximum steering torque limit, for example, 3 Newton-meters, for safety purposes. However, under certain driving conditions, such as driving at highway speeds and negotiating a turn having a relative tight radius of curvature, such as 500 meters, the steering torque limit is not high enough to maintain the vehicle in the lane. Particularly, under these types of conditions, the vehicle steering needs more than 3 Newton-meters of steering torque to stay within the lane. Because that amount of torque is beyond what the lane centering system can provide, the lane centering and/or lane keeping algorithms must address this situation. Current lane centering and lane keeping controllers often employ lane departure warnings when the vehicle begins traveling outside of the lane. Because the vehicle driver may not be fully attentive during autonomous vehicle driving, any lane departure warning given by the system when the vehicle is leaving the lane may be too late for the vehicle operator to take over control of the vehicle and stay within the lane. Therefore, there is a need in the art to provide a warning in advance of this situation so that the driver can adequately respond.
The discussion above refers to determining steering torque for lane keeping purposes. A typical lane keeping or lane changing control algorithm calculates vehicle steering angle as a command to steer the vehicle and not steering torque. However, most steering systems, for example, electrical power steering (EPS) systems, typically only accept steering torque as a command. Therefore, these systems require some technique for converting steering angle to steering torque. An inaccurate conversion of steering angle to steering torque may cause vehicle oscillations during a lane control maneuver. Current steering models are generally not applicable for converting steering angle to steering torque in lane control algorithms because of limited computing power, the non-existence of suitable sensors and the inaccuracy of the angle to torque conversions for stop and go situations. Thus, there is a need in the art to effectively convert a steering angle command to a steering torque command in these types of lane control systems.